Multiple doping level bipolar junctions transistors and method for forming

ABSTRACT

A process for forming bipolar junction transistors having a plurality of different collector doping densities on a semiconductor substrate and an integrated circuit comprising bipolar junction transistors having a plurality of different collector doping densities. A first group of the transistors are formed during formation of a triple well for use in providing triple well isolation for complementary metal oxide semiconductor field effect transistors also formed on the semiconductor substrate. Additional bipolar junction transistors with different collector doping densities are formed during a second doping step after forming a gate stack for the field effect transistors. Implant doping through bipolar transistor emitter windows forms bipolar transistors having different doping densities than the previously formed bipolar transistors. According to one embodiment of the present invention, bipolar junction transistors having six different collector dopant densities (and thus six different breakdown characteristics) are formed.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Divisional of prior application Ser. No.12/243,137 filed on Oct. 1, 2008 now U.S. Pat. No. 7,713,811, entitled“MULTIPLE DOPING LEVEL BIPOLAR JUNCTIONS TRANSISTORS AND METHOD FORFORMING” to Daniel Charles Kerr, et al. currently allowed, which is aDivisional of prior application Ser. No. 11/458,270 filed on Jul. 18,2006, entitled “MULTIPLE DOPING LEVEL BIPOLAR JUNCTIONS TRANSISTORS ANDMETHOD FOR FORMING,” to Daniel Charles Kerr, et al. U.S. Pat. No.7,449,388 issued on Nov. 11, 2008, which is a Divisional of U.S. Pat.No. 7,095,094 issued on Aug. 22, 2006 (Ser. No. 10/953,894 filed on Sep.29, 2004), entitled “MULTIPLE DOPING LEVEL BIPOLAR JUNCTIONS TRANSISTORSAND METHOD FOR FORMING,” to Daniel Charles Kerr, et al. The above-listedApplications are commonly assigned with the present invention andincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to bipolar junction transistorsand more specifically to a process for forming bipolar junctiontransistors having different collector doping levels and to transistorsformed according to the process.

BACKGROUND OF THE INVENTION

A bipolar junction transistor (BJT) comprises three adjacent dopedsemiconductor regions or layers having an NPN or PNP dopingconfiguration. A middle region forms a base and two end regions form anemitter and a collector. Typically, the emitter has a higher dopantconcentration than the base and the collector, and the base has a higherdopant concentration than the collector. Generally, the BJT can beoperated as an amplifier (for example, to amplify an input signalsupplied between the base and the emitter, with the output signalappearing across the emitter/collector) or as a switch (for example, aninput signal applied across the base/emitter switches theemitter/collector circuit to an opened or a closed (i.e.,short-circuited) state. In operation, the emitter/base pn junction isforward biased and the collector/base pn junction is reverse biased.

There are several known semiconductor fabrication processes for formingthe three doped layers of a bipolar junction transistor and severaltransistor architectures can be formed according to such processes. Thesimplest structure is a planar architecture with the stacked NPN or PNPregions formed by successive dopant implants into a semiconductorsubstrate.

A metal-oxide field effect transistor (MOSFET) differs in structure andoperation from a BJT. The MOSFET comprises source and drain dopedregions formed in a tub or well having an opposite doping type. Avoltage applied to a gate disposed above the well between the source anddrain inverts the conductivity of a channel region between the sourceand the drain, permitting current flow through the channel.

In certain MOSFET devices it may be desired to bias a p-doped tub of anNMOSFET separately from the substrate in which the tub is formed. Thisbias arrangement requires complete isolation of the entire p-tub fromthe substrate. One common isolation technique comprises junctionisolation, for example, a triple well process wherein an implant stepdopes n-type a region of the silicon substrate immediately below thep-tub. A subsequent n-type tub implant forms a ring of n-type materialaround the periphery of the p-tub, extending downwardly to connect withthe n-type triple well to isolate the p-tub from the substrate. Thetriple well and tub implant steps are performed prior to forming theMOSFET gate and its associated components.

Most integrated circuits for processing digital signals comprise aplurality of complimentary metal-oxide semiconductor field effecttransistors (CMOSFETS) each of which further comprises an arrangement ofan NMOSFET formed in a p-type substrate well and a PMOSFET formed in ann-type well. Certain of these integrated circuits also include bipolarjunction transistors and thus are referred to as BiCMOS circuits. CMOSdevices consume less power and occupy less integrated circuit area thanthe BJT. However, BJT's generally exhibit a higher switching speed.

It is known that there is a maximum reverse voltage that can be appliedacross any two junctions of the BJT before the junction breaks down.Break down current flow through the reverse-biased junction can generateexcess heat and may destroy the junction and thus the transistor. Thebreakdown voltage of a pn junction decreases as the doping level oneither or both sides of the junction increases. Thus, to increase thebreakdown voltage the doping level on one or both sides of the junctionis reduced. The lower dopant density lowers the electric field intensityacross the junction for a fixed applied voltage, or stated differently,raises the voltage at which junction breakdown occurs. However, thereare known disadvantages to low-doped junctions, including a higherjunction resistance and slower switching speed or response time to aninput signal.

The two BJT breakdown characteristics of interest are BV_(CEO)(collector-emitter breakdown voltage with the base open circuited) andBV_(CBO) (collector-base junction breakdown voltage with the emitteropen circuited). Since the collector current injected into the baseregion is multiplied by the transistor gain (β), these two breakdowncharacteristics are related byBV _(CEO) /BV _(CBO)≈(1/β)^(1/m)where m is between about 3 and 6.

Cost considerations constrain BiCMOS fabricators to offer one or at mosttwo BJT breakdown classes or types in an integrated circuit, i.e., afirst type exhibiting a relatively high breakdown voltage (i.e.,BV_(CEO) and BV_(CBO)) and a second type exhibiting a relatively lowbreakdown voltage. To form two BJT from each class in an integratedcircuit, a first mask is employed to expose certain collector regionswhere a first dopant concentration is implanted using an ion dose with afirst implant energy. A second mask exposes other collector regions forreceiving a second implant ion dose at a second implant energy to form asecond dopant concentration. Offering bipolar transistors with other(e.g., intermediate) breakdown voltages provides the circuit designerwith additional design flexibility, but requires developing another maskand adding additional implant and mask process steps, thereby increasingfabrication costs. Thus providing additional transistor breakdownclasses may not be justified by the extra cost of the masking andimplant steps.

Recognizing these cost limitations, during integrated circuit design,BJT breakdown characteristics are selected based on perceived customerneeds, and circuit designers are restricted to the selected BJTbreakdown classes fabricated in the integrated circuit. This “one sizefits all” compromise approach can cause difficulties in implementing acircuit with desired operational characteristics when the designer isrestricted to only two breakdown values for the BJT's on the integratedcircuit. It is thus desired to expand the number of NPN breakdownclasses without increasing the fabrication cost.

Each integrated circuit foundry employs specific fabrication processparameters that are unique to the foundry, e.g., wafer routing, processchecks, process recipes, process documentation, test conditions andspecification limits. As a result, difficulties may be encountered intransferring fabrication of an integrated circuit product (including aBJT) from one foundry to another, as the receiving foundry may notpossess the capability to manufacture the integrated circuit. Tosuccessfully transfer the product, the receiving foundry must match theprocess parameters of the transferring foundry, which can be a difficultand costly undertaking.

As related to fabrication of BJT's with different breakdown voltages,each foundry typically individually optimizes its NPN voltage breakdownofferings. Thus it is difficult to transfer a BJT product from onefoundry to another. A foundry capable of producing several different BJTbreakdown types is more likely to successfully fabricate a BiCMOSintegrated circuit transferred from another fabrication foundry.

BRIEF SUMMARY OF THE INVENTION

A process according to the present invention comprises forming a bipolarjunction transistor in a semiconductor substrate The process comprisesforming a first doped tub region of a first dopant type within thesubstrate, forming a doped sinker region of a second dopant type withinthe substrate and forming second and third doped tub regions of thesecond dopant type in the substrate. A subcollector region and a triplewell region both of the second dopant type are formed concurrently. Thetriple well region and the second and the third doped tub regionscooperate to electrically isolate the first doped tub region from thesubstrate. The subcollector region cooperates with the doped sinkerregion.

An apparatus according to the present invention comprises asemiconductor integrated circuit further comprising a plurality ofbipolar junction transistors disposed in a semiconductor substrate. Theintegrated circuit comprises a first subcollector region having a firstdoping level; (TW) a second subcollector region having a second dopinglevel; (NPN) a third subcollector region having a third doping level;(TW+NPN) a fourth subcollector region having the first doping level anda portion thereof having a fourth doping level; (TW+SIC) a fifthsubcollector region having the second doping level and a portion thereofhaving the fourth doping level; and (NPN+SIC) a sixth subcollectorregion having the third doping level and a portion thereof having thefourth doping level. (TW+NPN+SIC)

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention will be apparent fromthe following more particular description of the invention, asillustrated in the accompanying drawings, in which like referencecharacters refer to the same parts throughout the different figures. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

FIGS. 1-16 are cross-sectional illustrations of sequential processingsteps for forming a bipolar junction transistor and metal-oxidesemiconductor field effect transistors according to the presentinvention.

FIG. 17 is a graph illustrating an approximate doping density for abipolar junction transistor constructed according to the teachings ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before describing in detail the particular method and apparatus forforming a bipolar junction transistor on a semiconductor integratedcircuit, it should be observed that the present invention residesprimarily in a novel and non-obvious combination of elements and processsteps. So as not to obscure the disclosure with details that will bereadily apparent to those skilled in the art, certain conventionalelements and steps have been presented with lesser detail, while thedrawings and the specification describe in greater detail other elementsand steps pertinent to understanding the invention.

FIGS. 1-16 show cross-sectional views of formed structures according tosequential process steps for forming more than two breakdown classes forNPN bipolar junction transistors. The number of breakdown classes is atleast doubled over the prior art.

The illustrated process is a BiCMOS process wherein bipolar transistorsand CMOS devices are formed on a single substrate. NPN BJT's are formedin BJT regions 1-6 of a substrate 10 of FIG. 1, with each BJT having adifferent breakdown voltage, e.g., BV_(CEO) and BV_(CBO), due todifferent collector doping densities. An NMOSFET is formed in an NMOSFETregion 7 and a PMOSFET is formed in a PMOSFET region 8 of the substrate10. Although the invention is described in the context of a BiCMOSfabrication process and BiCMOS devices, those skilled in the artrecognize that the teachings of the present invention are not restrictedto BiCMOS integrated circuit embodiments, but can be employed tofabricate BJT's in embodiments other than in combination with CMOStransistors.

The present invention teaches a process for forming several classes ofBJT's and BJT structures formed according to the process, with eachclass having different breakdown characteristics. Although each class isrepresented by only one BJT in the following description andaccompanying figures, this is merely exemplary as an integrated circuittypically comprises a plurality of BJT's in each breakdown class. Also,in certain embodiments all breakdown classes may not be present in thesemiconductor integrated circuit.

To avoid performance degradation and electrical cross-talk between thedevices of an integrated circuit, it is necessary to electricallyisolate the BJT's and the CMOS devices. The illustrated exemplaryprocess employs LOCOS (local oxidation of silicon) isolation. To formthe LOCOS regions, the silicon substrate 10 is oxidized to form asilicon dioxide layer, also referred to as a pad oxide layer. A siliconnitride layer is deposited overlying the silicon dioxide layer. Thesilicon nitride layer and the silicon dioxide layer are etched to removecertain regions thereof according to an overlying patterned photoresistlayer. The remaining regions of the silicon dioxide and silicon nitridelayers serve as an oxidation mask during a LOCOS oxidation process, suchthat the isolation regions are formed in the substrate 10 only throughthe openings in the two layers. The LOCOS oxidation process formsisolation silicon dioxide regions 20 illustrated in FIG. 1.

It is known that in other embodiments, different isolation schemes canbe used, such as shallow trench isolation alone or a combination ofshallow and deep trench isolation.

After forming the isolation regions 20, the remaining regions of thesilicon nitride and the silicon dioxide layers are removed and asacrificial silicon dioxide layer 22 is formed over the substrate 10 inregions between adjacent isolation regions 20.

Next, a photoresist layer (not shown in FIG. 2) is deposited, masked,patterned and developed to form a p-tub photoresist implant mask. Ap-type dopant is implanted through the mask to form a p-tub 27 in theNMOS region 7 of the substrate 10. The p-tub 27 as formed according tothese process steps is illustrated in FIG. 2.

Continuing with FIG. 2, an n-tub photoresist implant mask 30 is formedand n-type dopants implanted through the mask, as indicated by implantarrowheads 32. The implant process forms an n-tub 40 in the PMOS region8, n-tubs 41A and 41B in the BJT region 1, n-tubs 42A and 42B in the BJTregion 2, n-tubs 43A and 43B in the BJT region 3, n-tubs 44A and 44B inthe BJT region 4, n-tubs 45A and 45B in the BJT region 5, n-tubs 46A and46B in the BJT region 6, and n-tubs 47A and 47B in NMOS region 7. Then-tubs associated with the BJT regions 1-6 operate as sinker regions forconnecting to subsequently formed subcollectors, as described below.

As illustrated in FIG. 3, a triple well mask 48 is formed over thesubstrate 10 and dopants implanted through the mask 48 as indicated byn-type dopant implant arrowheads 50, forming n-type subcollectors51A-51D in the BJT regions, 1, 2, 4 and 6, respectively, and an n-typetriple well region 55 below the p-tub 27. The subcollectors 51A-51D eachhave substantially the same dopant concentration and overlap with therespective n-tubs 41A/41B, 42A/42B, 44A/44B and 46A/46B as indicated toform a relatively contiguous doped region.

The implant step illustrated in FIG. 3, referred to as the triplewell/subcollector implant, serves a dual purpose. In the NMOS region 7,the implant step forms a deep n-well region for a triple well process.Overlapping dopants in the triple well region 55 with the tubs 47A and47B forms an n-type isolation tub around the p-tub or p-well 27. In theBJT regions, this triple well/subcollector dopant implant step formssubcollectors for low-breakdown BJT's as further described below.

According to prior art practice, formation of the subcollectors 51A-51Dprior to process steps forming the gate of the MOSFET devices results inundesired dopant diffusion in the subcollectors during subsequentthermal cycles, including gate oxidation thermal cycling. The presentinventors have determined that formation of the subcollectors 51A-51Dprior to gate processing is an effective alternative to the prior artpractice, and it has been determined that the BJT's formed according tothis practice have desired performance characteristics. Also accordingto the prior art, the implant dose forming the BJT subcollector is about20-50 times higher than the implant dose forming the triple well region55. For example, the triple well process of the prior art employs animplant dose of about 2E13 per cubic cm and the BJT subcollector processof the prior art employs a dose of about 5E14 per cubic cm. According tothe teachings of the present invention, the FIG. 3 triplewell/subcollector implant process that forms both the triple well andthe BJT subcollectors employs an implant dose of about 2E13 per cubiccm.

According to another prior art process, a high dose implant forms theBJT's, a blanket silicon epitaxial layer is formed and then the n-tuband the p-tub are formed by implant steps. The present invention differsfrom this practice by using the same process step to form both the BJTsubcollectors and the triple well.

A CMOS gate stack for the PMOS and NMOS devices is formed next accordingto known techniques. In an exemplary process, a wet clean processremoves the photoresist mask and the sacrificial silicon dioxide layer22. Gate stack layers comprising a silicon dioxide layer, a polysiliconlayer and a tungsten silicide layer are grown or blanket deposited on anupper surface 60 of the substrate 10. After doping the polysilicon layerthrough an implant mask, the gate stack layers are patterned, typicallyusing an oxide hard mask. FIG. 4 illustrates the resulting stackstructure, comprising a gate oxide region 61, a polysilicon region 62, atungsten silicide region 63 and an oxide hard mask layer 64, overlyingeach of the p-tub 27 (in the NMOS region 7) and the n-tub 40 (in thePMOS region 8).

Using a patterned implant mask 66 of FIG. 5, n-type dopants areimplanted (as represented by arrowheads 67) to form an n-type lightlydoped drain region 68 (NLDD) in the p-tub 27. N-type dopants are alsoimplanted in the BJT regions 1-6 to form n+ collector surface regions69, 70, 71, 72, 73 and 74, respectively.

Using another appropriately patterned implant mask, not shown in FIG. 5,a p-type lightly doped drain region 76 (PLDD) is formed in the n-tub 40.

A BJT implant mask 90 is formed overlying the substrate 10 and patternedas illustrated in FIG. 6. N-type implant ions, as represented byarrowheads 92, are implanted during an NPN doping process to modify thedopant concentration of the BJT regions 3-6. In the BJT regions 4 and 6,the NPN doping process adds dopants to the existing dopantconcentrations to form subcollector doped regions 96A and 96B, havingsubstantially equal dopant concentrations. In previously undoped regions3 and 5 (i.e., regions 3 and 5 lacking previous subcollector doping) theNPN implant dose forms subcollector doped regions 94A and 94B havingsubstantially equal dopant concentrations. Since the subcollector dopedregions 96A and 96B were previously doped, the dopant concentration ofthe subcollector doped regions 96A/96B is different from the dopantconcentration of the subcollector doped regions 94A/94B.

Since the BJT regions 1 and 2 are masked during the FIG. 6 NPN dopingprocess, these regions retain their dopant concentrations according tothe triple well doping process of FIG. 3.

According to a preferred embodiment of the present invention, the FIG. 6NPN implant dose is greater than the FIG. 3 triple well/subcollectorimplant dose, resulting in a higher dopant concentration in thesubcollector doped regions 94A/94B than in the subcollector dopedregions 51A/51B.

At this point in the fabrication process, the formed subcollectorregions have one of three different dopant concentrations. The BJTregions 1 and 2 have a first dopant concentration from the triplewell/subcollector implant of FIG. 3. The BJT regions 3 and 5 have asecond dopant concentration from the NPN doping process of FIG. 6. TheBJT regions 4 and 6 have a third dopant concentration from a combinationof the triple well/subcollector doping process and the NPN dopingprocess. Since the subcollector doping concentration is directly relatedto the transistor's breakdown voltages BV_(CEO) and BV_(CBO), BJT's withthree different breakdown characteristics have been formed.

A TEOS (tetraethylorthosilicate) silicon dioxide layer 110 and apolysilicon layer 112 are formed overlying the substrate 10. See FIG. 7.The polysilicon layer 112, which will form an extrinsic base region ofthe BJT's, is doped with a high dose boron implant, not shown.

Next a silicon nitride layer 120 and a silicon dioxide layer 122 (in oneembodiment formed according to a TEOS process) are deposited over thepolysilicon layer 112. A photoresist layer is deposited and patternedfor use in anisotropically etching the silicon dioxide layer 122, thesilicon nitride layer 120 and the polysilicon layer 112 to form emitterwindows 130 for the BJT's 1-6. The emitter windows 130 stop on thesilicon dioxide layer 110.

The photoresist layer is removed and a selectively implanted collector(SIC) photoresist layer 150 is formed over the substrate 10 as furtherillustrated in FIG. 7. As indicated by implant arrowheads 152, n-typedopants are implanted into the exposed BJT regions 2, 5 and 6immediately below each exposed emitter window 130, forming doped regions153, 154 and 155 in the BJT regions 2, 5 and 6. The SIC implant does notpenetrate through the combined thickness of the silicon dioxide layer110, the polysilicon layer 112, the silicon nitride layer 120 and thesilicon dioxide layer 122.

Since the SIC implant is relatively shallow, the implant dopantspenetrate only through the silicon dioxide layer 110 and into a regionimmediately below the emitter windows 130, forming the SIC doped regions153 (for the BJT 2), 154 (for the BJT 5) and 155 (for the BJT 6). It isknown that the doping concentration at the collector base junction,which is immediately below the silicon dioxide region underlying theemitter window 130, contributes to the transistor breakdowncharacteristics.

The collector regions of the BJT's 1-6 formed to this point in theprocess have one of six different dopant concentrations. The collectorof the BJT region 1 has a first dopant concentration from the triplewell/subcollector implant of FIG. 3. The collector of the BJT region 2has a second dopant concentration derived from a sum of the triplewell/subcollector implant of FIG. 3 and the SIC doping process of FIG.7. The collector of the BJT region 3 has a third dopant concentrationfrom the NPN doping process of FIG. 6. The collector of the BJT region 4has a fourth dopant concentration that is a sum of the triplewell/subcollector and the NPN doping processes. The collector of the BJTregion 5 has a fifth dopant concentration from the NPN doping of FIG. 6and the SIC implant of FIG. 7. The collector of the BJT region 6 has asixth dopant concentration from the triple well/subcollector dopingprocess of FIG. 3, the NPN doping process of FIG. 6 and the SIC implantof FIG. 7.

From this point in the process, all of the BJT's 1-6 are exposed toidentical fabrication process steps. Thus for the purpose of simplifyingthe figures and the descriptive text, the subsequent process steps aredescribed for only the BJT region 1.

A layer of silicon nitride is deposited overlying the substrate 10 andanisotropically etched to form sidewall spacers 162 within the emitterwindow 130 as shown in FIG. 8. A wet etch process removes the silicondioxide layer 110 from within the emitter window 130, forming a cavity168 and cavities 170 (see FIG. 9) laterally disposed relative to thecavity 168.

An intrinsic base and a cap region, both identified by a referencecharacter 176 (see FIG. 10) are formed in the cavities 168 and 170during a selective silicon-germanium epitaxial growth step.

Spacers 178, comprising a silicon nitride layer 178A and an underlyingsilicon dioxide layer 178B, are formed over the spacers 162, asillustrated in FIG. 11. The spacers 178, which increase the distancebetween a later-formed n+emitter and a p+ extrinsic base in the window130, are formed by depositing and anisotropically etching a TEOS silicondioxide layer and a silicon nitride layer. In another embodiment, thespacers 178 may not be required as the previously-formed spacers 162 aresufficient to provide the necessary separation between the emitter andextrinsic base.

To form an emitter 180 of FIG. 12, a polysilicon layer is deposited(including deposition within the emitter window 130) and doped n-type byimplant or in-situ doping. A hard mask and photoresist mask are utilizedto pattern the doped polysilicon layer to form the emitter 180. Regionsof the silicon nitride layer 120 are also removed during the emitterregion etch, leaving silicon nitride regions 120A underlying the emitterregion 180. Using another photoresist mask, the polysilicon layer 112 isetched to form an extrinsic base 112B as shown in FIG. 13.

At this point processing of the BJT's is essentially complete; each BJTcomprises the intrinsic base 176, the extrinsic base 112B (formed fromdoped polysilicon), the emitter 180 (formed from doped polysilicon) thecollector regions as follows: for the BJT region 1 the collector regioncomprises the tubs 41A and 41B and the triple well 51A; for the BJTregion 2 the collector regions comprises the tubs 42A and 42B, thetriple well 51B and the SIC doped region 153; for the BJT region 3 thecollector region comprises the tubs 43A and 43B and doped region 94A;for the BJT region 4 the collector region comprises the tubs 44A and 44Band the doped region 96A (further comprising the triple well 51C plusdopants added by the doping process illustrated in FIG. 6); for the BJTregion 5 the collector region comprises the tubs 45A and 45B, the dopedregion 94B and the SIC doped region 154 and for the BJT region 6 thecollector region comprises the tubs 46A and 46B and the doped region 96B(further comprising the triple well 51D plus dopants added by the dopingprocess of FIG. 6) and the SIC doped region 155. The collector region ofeach of the BJT's regions 1-6 further comprises the collector surfaceregions 69-74, respectively, as shown in FIG. 7.

Continuing with formation of the MOSFETS, during an anisotropic etchstep, the silicon dioxide layer 110 is etched to form spacers 200 (seeFIG. 14, which depicts a partial view of the substrate 10, illustratingonly the NMOSFET 7 and the PMOSFET 8) adjacent the gate stacks, eachgate stack comprising the gate oxide region 61, the polysilicon region62, the tungsten silicide region 64 and the hard mask region 64. Thisetch step also removes the silicon dioxide layer 110 from the BJTregions 1-6, forming silicon dioxide regions 110A. FIG. 15 illustratesthe resulting structure for the exemplary BJT region 1.

Conventional processing steps are executed for completing fabrication ofa PMOSFET 220 and an NMOSFET 222 illustrated in FIG. 16, includingimplanting source/drain regions 230 in the p-tub 27 (for the NMOSFET222) and source/drain regions 234 in the n-tub 40 for the PMOSFET 220.After forming contact regions for the doped regions of the BJT's 1-6,the PMOSFET 220 and the NMOSFET 222, an interconnect structure,comprising alternating dielectric and conductive layers, is formed overthe substrate 10 according to known processes. Windows are defined in afirst dielectric layer and conductive plugs formed therein forestablishing electrical contact with contact regions of the BJT's 1-6,the PMOSFET 220 and the NMOSFET 222. Interconnect structures forconnecting the regions are formed in a first metallization layer.Additional alternating dielectric layers and interconnect layers areformed over the first metallization layer to complete the interconnectsystem of the integrated circuit. Interconnect structures can be formedby conventional metal deposition and subtractive etching processes or bydamascene processes.

Exemplary implant values to produce the various BJT doped regions asdescribed herein are set forth below:

for the triple well implant described in conjunction with FIG. 3:phosphorous at about 1000 keV and a dose of about 2E13 per cubic cm;

for the NPN implant described in conjunction with FIG. 6: phosphorous atabout 900 keV and a dose of about 5E14 per cubic cm;

for the SIC implant described in conjunction with FIG. 7: phosphorous atabout 500 keV and a dose of about 1.3E13 per cubic cm and phosphorous atabout 190 keV and a dose of about 4E12 per cubic cm. In certainembodiments, both SIC implants are used to establish the breakdowneffects described above.

FIG. 17 illustrates an exemplary relative doping profile as a functionof depth for the BJT's 1-6. The BJT doped regions and the typical dopantfor each region are indicated in FIG. 17. An arrowhead 230 identifies aregion (of about 1-2 microns in depth) in the collector where the dopinglevel primarily determines the breakdown voltage BV_(CEO) and BV_(CBO)of the BJT.

A dopant density 232 represents the dopant density generally in thesubcollector of the BJT region 3 as formed by the NPN doping process ofFIG. 6. A dopant density 234 in the BJT region 1 results from the triplewell/subcollector doping process of FIG. 3. A dopant density 236represents the dopant density for the BJT region 4 that has beensubjected to both the NPN and the triple well/subcollector dopingprocesses. A dopant density 238 in the BJT region 2 represents thedopant density in the collector region as formed by the triplewell/subcollector doping process of FIG. 3 and the SIC doping process ofFIG. 7. A dopant density 240 in the BJT region 5 results from the NPNand the SIC doping processes. A dopant density 244 represents the dopantdensity for the BJT region 6 that has been subjected to the NPN, triplewell/subcollector and SIC doping processes. Since each of the BJTregions 1-6 has a different collector doping density, each exhibits adifferent breakdown voltages, where a higher dopant density produces ahigher breakdown voltage. Table 1 below summarizes the dopant densitiesof the BJT regions 1-6.

TABLE 1 BJT REGION DOPANT DENSITY DERIVED FROM 1 Triplewell/subcollector doping of FIG. 3 2 Triple well/subcollector dopingplus SIC doping of FIG. 7 3 NPN doping of FIG. 6 4 Triplewell/subcollector doping plus NPN doping 5 NPN doping plus SIC doping 6Triple well/subcollector doping plus NPN doping plus SIC doping

The teachings of the present invention can also be applied to BJT'sfabricated from a single polysilicon layer. In this embodiment, a baseregion is formed in the substrate overlying the collector region. Theemitter is formed from the polysilicon layer deposited within theemitter window as described above. The teachings can also be applied totransistor structures where the subcollector is not implanted (i.e., arelatively shallow subcollector implant and a relatively thick siliconepitaxial layer formed before any other processing steps are executed).Further, the teachings can be applied to a transistor having onlyimplanted regions or junctions (i.e., without using polysilicon layersto form the transistor regions).

An architecture and process have been described as useful for formingbipolar junction transistors in a semiconductor substrate. Specificapplications and exemplary embodiments of the invention have beenillustrated and discussed, and provide a basis for practicing theinvention in a variety of ways and in a variety of circuit structures.Numerous variations are possible within the scope of the invention.Features and elements associated with one or more of the describedembodiments are not to be construed as required elements for allembodiments. The invention is limited only by the claims that follow.

1. A process for forming bipolar junction transistors and metal oxidesemiconductor field effect transistors over a substrate, the processcomprising: forming a first doped tub and a plurality of doped collectorsinker regions within the substrate; forming a second and a third dopedtub within the substrate; forming a triple well region for cooperatingwith the second and the third doped tubs to electrically isolate thefirst doped tub from the substrate and forming a first plurality ofsubcollector regions comprising a first doping level; forming structuresassociated with the metal oxide semiconductor field effect transistor;doping a first subset of the first plurality of subcollector regions toform a second plurality of subcollector regions while doping otherregions of the substrate to form a third plurality of subcollectorregions, wherein the second plurality of subcollector regions comprise asecond doping level and the third plurality of subcollector regionscomprise a third doping level; and doping a second subset of the firstplurality of subcollector regions to form a fourth plurality ofsubcollector regions while doping a subset of the third plurality ofsubcollector regions to form a fifth plurality of subcollector regionsand while doping a subset of the first subset of the first plurality ofsubcollector regions to form a sixth plurality of subcollector regions,wherein the fourth plurality of subcollector regions comprise a fourthdoping level, and wherein the fifth plurality of subcollector regionscomprise a fifth doping level, and wherein the sixth plurality ofsubcollector regions comprise a sixth doping level.
 2. The process ofclaim 1 further comprising forming a base region and an emitter over thesubcollector and doped sinker region in each of the first, second,third, and fourth BJT regions.
 3. The process of claim 2 wherein BJT inthe first, second, third, and fourth BJT regions exhibit differentbreakdown characteristics according their respective doping levels. 4.The process of claim 1 wherein forming structures associated with themetal oxide semiconductor field effect transistor further comprisesdepositing a layer of polysilicon and forming a gate for the metal oxidesemiconductor field effect transistor.
 5. The process of claim 1,wherein the subcollectors in the first, second and fourth BJT regionsand the triple wells in the first MOSFET region are formed using a firstmask.
 6. The process of claim 5, wherein concurrently doping unmaskedportions of the doped sinker regions in the first, second, third andfourth BJT regions and the first doped tub regions with the second typedopant includes using a second mask different from the first mask. 7.The process of claim 6, wherein doping the third and fourth BJT regionswith the second dopant type to form a subcollector in the third BJTregion and further doping the subcollector in the fourth BJT regionincludes using a third mask different from the first and second masks.8. A process for forming bipolar junction transistors in a semiconductorsubstrate further comprising a metal oxide semiconductor field effecttransistor, the process comprising: forming a first doped tub and aplurality of doped collector sinker regions within the substrate;forming a second and a third doped tub within the substrate; forming atriple well region for cooperating with the second and the third dopedtubs to electrically isolate the first doped tub from the substrate andforming a first plurality of subcollector regions comprising a firstdoping level; forming structures associated with the metal oxidesemiconductor field effect transistor; doping a first subset of thefirst plurality of subcollector regions to form a second plurality ofsubcollector regions while doping other regions of the substrate to forma third plurality of subcollector regions, wherein the second pluralityof subcollector regions comprise a second doping level and the thirdplurality of subcollector regions comprise a third doping level; anddoping a second subset of the first plurality of subcollector regions toform a fourth plurality of subcollector regions while doping a subset ofthe third plurality of subcollector regions to form a fifth plurality ofsubcollector regions and while doping a subset of the first subset ofthe first plurality of subcollector regions to form a sixth plurality ofsubcollector regions, wherein the fourth plurality of subcollectorregions comprise a fourth doping level, and wherein the fifth pluralityof subcollector regions comprise a fifth doping level, and wherein thesixth plurality of subcollector regions comprise a sixth doping level.9. The process of claim 8 further comprising forming a base region andan emitter region for cooperating with one of the first, second, third,fourth, fifth and sixth subcollector region and one of the plurality ofdoped collector sinker regions to form, respectively, a first, second,third, fourth, fifth and sixth bipolar junction transistor.
 10. Theprocess of claim 9 wherein the first, second, third, fourth, fifth andsixth bipolar junction transistors exhibit breakdown characteristics inthe following order beginning with the bipolar junction transistorhaving the highest breakdown voltage, according to the doping level ofthe subcollector, the first doping level, the fourth doping level, thethird doping level, the second doping level, the fifth doping level andthe sixth doping level.
 11. The process of claim 8 wherein the step offorming structures associated with the metal oxide semiconductor fieldeffect transistor further comprises depositing a layer of polysiliconand forming a gate for the metal oxide semiconductor field effecttransistor.
 12. The process of claim 8 wherein the structures associatedwith the metal oxide semiconductor field effect transistor comprise agate structure, and wherein the step of forming structures associatedwith the metal oxide semiconductor field effect transistor is executedafter the step of forming the triple well region and the first pluralityof subcollector regions and before the step of doping the first subsetof the first plurality of subcollector regions.
 13. The process of claim12 wherein the step of forming structures associated with the metaloxide semiconductor field effect transistor is executed before the stepof doping the first subset of the first plurality of subcollectorregions.
 14. The process of claim 8 further comprising forming materiallayers overlying the substrate and forming emitter windows within one ormore of the material layers, wherein the step of doping the secondsubset of the first plurality of subcollector regions while doping asubset of the third plurality of subcollector regions and while doping asubset of the first subset of the first plurality of subcollectorregions comprises doping through the emitter windows.